1. Field of the Invention
The present invention relates to the field of antenna-based bolometric detectors and, more especially, detectors with two crossed bow-tie antennas designed to detect electromagnetic radiation in the region extending from infrared and, in particular the 3-5 μm and 8-14 μm bands, to terahertz frequencies.
2. Description of Related Art
Detection in the infrared range has many applications which are already widely known. As for terahertz frequencies, i.e. the range of frequencies from 100 GHz to 10 terahertz, envisaged applications include the following (this list is not exhaustive):                medical diagnostics where terahertz detection gives access to details of anatomical structure and chemical reactions which take place therein; neither x-rays or ultrasound provide this information;        military and flight safety applications, e.g. the construction of anti-stealth radar systems or high-resolution radars which provide good target discrimination;        studying and detecting atmospheric pollution; submillimeter-wave observations provide important information on the chemistry of the atmosphere, thus enabling unrivalled monitoring of airborne pollutants such as dinitrogen trioxide, for example, which is difficult to detect using conventional techniques because it has strong absorption lines in the far infrared region;        identification of chemical species; many complex chemical compounds have a signature in the terahertz region which is sufficiently unique to allow their reliable detection, for example, certain explosives and toxic products, some compounds which are released as fruits ripen and even some compounds released by industrial combustion;        analysis of phenomena at a molecular or atomic level; terahertz spectroscopy makes it possible to obtain new information on mechanisms such as photoexcitation, photodissociation and solvation. The same applies to the analysis of molecular interactions (vibratory states of molecules or hydrogen bonds for instance), condensed phase systems, dynamic processes in large molecules such as peptides and proteins and even observing and orienting polymers using a technique based on terahertz radiation;        studying the properties of materials such as semiconductors in order to determine, non-destructively, their mobility, for example, the dynamics of ultra-fast carriers and carrier-phonon interactions, supraconductors, polymers, ceramics, organic materials and porous materials. Moreover, materials such as plastics, paper and textiles are transparent in the terahertz region, metals are perfect reflectors and water is highly absorptive. Thus, detection in this region is especially suitable for inspecting packaged products or monitoring manufacturing processes in situ in real-time, for example; and        broadband telecommunications; the trend towards ever higher data rates, for terrestrial communications and communications between satellites, is encouraging manufacturers to develop systems which operate at frequencies which are already several hundred gigahertz and in the future may be as high as several terahertz.        
Usually, a resistive bolometric detector measures the power of incident radiation in the infrared region and, to achieve this, comprises an absorbing resistive bolometer element which converts the luminous flux into a heat flow which causes the temperature of said element to rise relative to a reference temperature. This increase in temperature then induces a change in the electrical resistance of the absorbing element which causes variations in the voltage or current across the latter's terminals. These electrical variations constitute the signal which is output by the detector.
However, the temperature of the absorbing element usually depends largely on its environment, especially that of the substrate which comprises the electronic readout circuit. In order to make the absorbing element as insensitive as possible to its environment, thereby improving the sensitivity of the detector, the absorbing element is generally thermally isolated from the substrate.
FIG. 1 is a schematic perspective view of an elementary resistive bolometric detector 10 according to the prior art, showing the principle of this thermal isolation. This elementary detector, shown here in the form of a suspended membrane, is classically part of a one or two dimensional array of elementary detectors.
Detector 10 comprises a thin membrane 12 which absorbs incident radiation and is suspended above substrate support 14 by two conductive posts 16 to which it is attached by two thermally isolating support arms 18. Membrane 12 usually comprises an electrically insulating layer made of, for example, SiO2, SiO, SiN, ZnS or another material which lends membrane 12 its mechanical rigidity, and a metal electrical interconnect layer deposited on the insulating layer.
A thin layer 20 of resistive thermometric material is also deposited in the centre of membrane 12 on the metal interconnect layer, especially a layer of a semiconductor material such as weakly or highly resistive polycrystalline or amorphous p- or n-type silicon or an oxide of vanadium (V2O5, VO2) produced in a semiconducting phase.
Finally, substrate support 14 comprises an electronic circuit incorporated in a silicon wafer which is usually referred to as a “readout circuit”. The readout circuit comprises, firstly, elements to excite (stimulate) and read thermometric element 20 and, secondly, multiplexing components which make it possible to serialize the signals obtained from the various thermometric elements contained in the array detector.
During operation, membrane 12 warms up due to the effect of incident electromagnetic radiation and the calorific power produced is transferred to the layer of thermometric material 20. The readout circuit in substrate 14 periodically biases membrane 12 by subjecting posts 16 to a bias voltage and taps off the current flowing through thermometric element 20 in order to deduce the variation in its resistance from this and hence the incident radiation which produced said variation.
Because the layout and operation of such a detector is conventional, it is not explained in further detail here for the sake of conciseness. It must, however, be noted that membrane 12 has three main functions apart from thermal isolation: An antenna function in order to receive radiation, a conversion function to convert received electromagnetic power into calorific power and a thermometric measurement function to measure the produced calorific power. Because it acts as an antenna, the dimensions of membrane 12 are consequently selected so that they are of the same order of magnitude as the wavelength of the radiation that is to be measured.
In the terahertz region, wavelengths can be as much as 1 mm and this therefore requires a membrane of roughly the same size. However, with such dimensions, the calorific mass, mechanical strength and radiation losses of the membrane are so problematic that they ultimately have an adverse effect on the efficiency of the detector.
This is why, for such a frequency range, the radiation receiver function is separated from the other functions. The radiation receiver function is thus fulfilled by a planar antenna and the function to convert electromagnetic power into calorific power is fulfilled by the antenna's resistive load. The dimensions of the load classically meet the requisite impedance matching conditions, which depend on the geometry of the antenna and the nature of the layers which support it, in order to obtain optimal conversion. The resistive load is also in thermal contact with a thermometric element used to measure the calorific power produced. This assembly then constitutes a bolometer with an antenna.
In such a configuration, the thermometric element is independent of the antenna and its size therefore no longer depends on the incident wavelength but on factors which determine the inherent performance of the detector (sensitivity, signal-to-noise ratio, etc.) appropriately, depending on the requirements of the application in question, active imaging or passive imaging for example.
In addition, in most cases, the incident electromagnetic radiation is not polarized so receiving it using just one antenna therefore makes it impossible to pick up all the available electromagnetic power. Nevertheless, non-polarized radiation can be regarded as two orthogonally superimposed, linearly polarized components, with each of these components transporting half the energy of the wave. As is well-known in itself, one efficient way of picking up incident electromagnetic radiation is to use two crossed bow-tie antennas. The bow-tie antenna is explained, for example, in the doctoral thesis with the following citation:
PEREZ Rafaël. Contribution à l'analyse théorique et expérimentale de radargrammes GPR: performances des antennes: apports d'une configuration multistatique [En ligne]. Thèse de doctorat: Électronique des Hautes Fréquences et Optoélectronique. Limoges: Université de Limoges, 2005.
Document U.S. Pat. No. 6,329,655 describes an antenna-based bolometer 30 which operates in the millimeter range and is fitted with two crossed bow-tie antennas 32, 34; schematic top and cross-sectional views of this bolometer are shown in FIGS. 2 and 3 respectively.
The principle of bolometer 30 is based on capacitive coupling between antennas 32, 34 located above substrate support 36 and a resistive load 38 located in a suspended membrane 40 on which thermometric element 42 is placed (FIG. 3).
Resistive load 38, which takes the form of a square layer positioned over the centre of antennas 32, 34, actually has a surface that faces the antennas and therefore forms a capacitance together with the antennas. The radiation picked up by antennas 32, 34 is thus transferred to load 38 by capacitive coupling.
However, the shape of the resistive load poses problems in terms of impedance matching.
It is estimated that impedance matching, and hence capacitive coupling, is actually optimal for this load when the following equation is satisfied:
                              (                      1                          π              ·              f              ·              C                                )                2            +                        (                      R            ·            C                    )                2              ≈      100    ⁢                  ⁢    Ω  where f is the frequency of the radiation, C is the value of the capacitance formed between antennas 32, 34 and resistive load 38, and R is the value of the resistance of resistive load 38.
Increasing the value of capacitance C to achieve optimum impedance matching or coupling is pointless because this assumes that there is either a submicron gap between antennas 32, 34 and load 38 or a large overlap surface area between them.
However, reducing the distance between the antennas and the load to a value of 100 to 200 nm poses difficulties in terms of physical phenomena (Casimir effect affecting mechanical stability, significant radiative heat transfer resulting in deterioration in thermal isolation of the thermoelectric element and hence a drop in the sensitivity of the detector) as well as current fabrication techniques (control of residual stresses in layers to prevent unwanted contacts or control of sacrificial layers used to form the gap between the antennas and the resistive load).
Not only that, increasing the size of the resistive load in order to increase the opposite-facing surfaces presents exactly the same problems as those that led one to separate the receive function and the conversion and thermometric functions as stated earlier. Consequently, to adapt the device for the terahertz domain, the value of capacitance C is not freely selectable.
In such a configuration, resistive load 38 must be weakly resistive, i.e. a sheet resistance of 50Ω to 200Ω, in order to ensure optimal coupling compatible with terahertz radiation. Due to inherent design features, this results in unwanted, practically optimal coupling with the infrared radiation emitted by bodies at 300° K. which is very difficult to eliminate effectively without impairing the quality of the signal in the terahertz frequency range.
Because of this, it is difficult to obtain optimum impedance matching and capacitive coupling using a resistive load associated with two antennas and having a square shape in the centre of the detector without, at the same time, also making coupling of the detector with infrared radiation optimal.
Moreover, because the two antennas 32, 34 are both placed on substrate 36, they are coupled via the substrate. It is observed that the gain of the detector is substantially reduced, thus making its use relatively unsatisfactory. For electrical connection reasons, especially the ease with which contact points can be fabricated between the readout circuit in the substrate and the thermometric element of the membrane, the substrate is usually as thin as possible. However, because this thickness e must satisfy the equation e=λ/4 n where n=√{square root over (∈)}, the permittivity ∈ will be very high and this will increase coupling between the two crossed antennas. Thus, it is not possible to reduce decoupling between the antennas by choosing the substrate accordingly without also significantly modifying the layout and operation of the electronic elements integrated in the substrate.